Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor. The rotor typically includes a rotatable hub having one or more rotor blades attached thereto. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have a cylindrical blade root that transitions to an airfoil shape towards a blade tip. The cross-sectional profile of the airfoil shape is such that, during operation, air flows from a leading edge to a trailing edge of the blade producing a pressure difference between a pressure side surface and a suction side surface, thereby creating a lift force. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity.
Ideally, the air flow is attached to the suction side surface from the leading edge to the trailing edge. However, when the angle of attack of the air flow exceeds a certain critical angle, the flow does not reach the trailing edge, but leaves the suction side surface at a flow separation line, which decreases potential energy production. Flow separation depends on a number of factors, such as incoming air flow characteristics (e.g. Reynolds number, wind speed, in-flow atmospheric turbulence) and characteristics of the blade (e.g. airfoil sections, blade chord and thickness, twist distribution, pitch angle, etc.). For example, flow separation tends to be more prevalent near the cylindrical blade root due to the relatively great angle of attack of the blade flow surfaces in this region as compared to the blade tip. The detached-flow region also leads to an increase in drag force, mainly due to an increase in profile wake and the related pressure drag (i.e. the attached flow state creates less pressure difference in the flow direction than the detached flow state). Hence, in order to increase the energy conversion efficiency during normal operation of the wind turbine, it is desired to increase the lift force of the blades while decreasing the drag force. To this purpose, it is advantageous to increase the attached-flow region and to reduce the detached-flow region by moving flow separation nearer the trailing edge of the blade.
It is known in the art to change the aerodynamic characteristics of wind turbine blades by adding dimples, protrusions, or other structures on the surface of the blade. These structures are often referred to as “vortex generators” and serve to create local regions of turbulent airflow over the surface of the blade as a means to preclude flow separation and thus optimize aerodynamic airflow around the blade contour. Such vortex generators, dimples, protrusions, and other similar features on the surface of the blade may be generally referred to herein as “airflow modifying elements.” Conventional vortex generators typically consist of a base having one or more raised surfaces and are attached to the suction side of the blade using adhesive tape. However, such airflow modifying elements must be properly aligned and may increase installation time and costs.
It is also known in the art to install blade fairings, flaps and/or other suitable extensions to the rotor blade, such as the cylindrical blade root, to improve aerodynamic characteristics of the blade in these areas. Such additions are typically mounted on the rotor blades to increase the rotor blade surface area and/or to improve aerodynamic performance and thus increase the lift. However, such additions must have a suitable thickness to withstand buckling and therefore add weight to the rotor blade that can negatively impact the performance of the rotor blade and the wind turbine in general. Further, the additions and/or fairings increase loading of the rotor blade and the downstream wind turbine components. In addition, manufacturing of the fairings can be expensive.
Thus, an improved rotor blade assembly for a wind turbine that addresses the aforementioned problems would be advantageous. For example, a rotor blade assembly that includes a blade root extension with one or more blade fences configured to improve aerodynamic performance of the rotor blade would be desired in the art.